The status of the middle ear system (tympanic membrane and ossicular chain) may be clinically ascertained by measuring aural acoustic immittance (AAI) at the entrance to the ear canal (immittance refers to either admittance or impedance). Typically, AAI measures are made as air pressure in the ear canal is parametrically varied below or above atmospheric pressure. AAI measures are obtained by sealing the tip of a probe, surrounded by a flexible cuff, in the opening to an ear canal, and the probe includes an air line through which pressure changes may be introduced. Ear canal pressure is monitored via a pressure transducer, and a control loop may be used to maintain or vary the pressure. Two commonly employed middle ear assessment tests based on AAI measures are tympanometry and acoustic reflex testing. In tympanometry, aural acoustic immittance is measured as air pressure in the ear canal is parametrically varied (e.g., +200 to −300 daPa) and a plot of immittance versus ear canal pressure during the pressure sweep is referred to as a tympanogram. The tympanogram provides a means to indirectly measure pressure in the middle ear cavity, since maximal admittance (or minimal impedance) occurs when ear canal pressure is equal to middle ear pressure. During acoustic reflex testing, ear canal pressure is maintained at the value that produced maximal admittance (minimal impedance) as inferred from the tympanogram, and changes in AAI are monitored as acoustic reflex eliciting stimuli are presented.
Thus, an ear canal pneumatic system suitable for AAI measurement requirements must be capable of providing an ear canal pressure sweep during tympanometric testing and of maintaining a static ear canal pressure during acoustic reflex testing. Noise produced by the pressure generation mechanism must be minimal, or the noise may be detected by the AAI measurement system and be misinterpreted as an admittance change, particularly during acoustic reflex testing. Ideally, an ear canal pressurization system should employ a reliable and easily implemented drive means and should yield a linear pressure versus drive function to ensure control loop stability. Lastly, an ideal ear canal pressurization system should have a long and maintenance-free life span.
AAI instrument pneumatic systems typically utilize syringe/plunger systems, oscillating diaphragm pumps, or peristaltic pumps. None of these systems meet all of the above stated requirements. Syringe/plunger systems are driven by stepper motors with rotary-to-linear gear drives. They require a stepper motor controller, acoustic damping in the air line to reduce stepper motor noise, and are prone to leakage due to plunger seal wear. Syringe/plunger systems also require valves to allow the plunger to be repositioned when plunger extents are reached before target pressure is achieved (e.g., in the presence of an air leak). Oscillating diaphragm pumps are flow-governed and can recover from a slight air leak, once the leak is sealed, without the need for “reset” valves, but they require considerable acoustic damping in the air line and separate pressure and vacuum pumps, connected via an air-flow restrictor. Oscillating diaphragm pumps also require tuning, since they perform optimally at a specific drive frequency, and this necessitates a more complicated calibration procedure. Peristaltic (“squeezed tube”) pumps utilize a stepper motor driven roller to produce air pressure by rolling over and compressing a flexible tube so that a small quantity of air is trapped and moved in one direction or the other. These pumps require acoustic damping in the air line and they also require periodic replacement of the tubing, which is constantly stressed by the roller providing the “squeeze”. Additionally, peristaltic pumps are prone to leaks when the spring force which holds the roller against the flexible tubing fails to fully pinch off the tubing. U.S. Pat. App. No. 2006/0197412 discloses an ear canal pressurization pump driven by a piezo electric motor. Such a pump would be quiet, since drive oscillations occur at a frequency above the range of human hearing, but the disclosed pump requires controlled valves to produce bidirectional pressure changes. Another disadvantage of piezo electric motors is that they require high frequency, high voltage driving signals and relatively sophisticated controllers.